Fuel is saved by shutting the gasoline engine down altogether whenever the vehicle comes to a full stop.
Also, since the electric is there to provide additional thrust, the gasoline engine doesn't need to be as large. (The Aura Green Line, for instance, has a 4-cylinder engine where non-hybrid versions have V6's.)
The advantages of a system like this are cost and size. The system requires little alteration to the basic engine and transmission layout so that it costs much less to manufacture than a complex "full hybrid" system.
Also, since electricity needs are lower, the system requires only a small battery pack which, again, saves on costs, but also saves weight and space. Other hybrid sedans, such as the Nissan Maxima and Toyota Camry Hybrids, lose trunk space to make room for batteries.
The downsides are that the system's impact on fuel economy is relatively small while the drag on driving performance is sizable.
When not boosting performance, the Green Line electric motor/generator remains fully engaged, acting like an anchor bogging the car down whenever the gas pedal isn't being pressed.